(Reuters) - U.S. drug reviewers said Pfizer's Prevnar 13 vaccine was effective for protecting adults over age 50 from pneumococcal infections, paving the way for a positive vote by outside experts later this week.

Reviewers from the Food and Drug Administration said trials for the vaccine also showed it did not have more safety risks than an older vaccine, according to documents posted on Monday.

The vaccine is being reviewed under the FDA's accelerated approval process, meaning the agency believes the medicine represents an unmet medical need.

A panel of outside experts to the FDA will consider whether to recommend approval of the drug during a meeting on Wednesday, and the FDA will make a final decision in January.

Two trials from Pfizer tested Prevnar 13 against the standard vaccine, Merck & Co's Pneumovax, which is currently the only vaccine for pneumococcal bacteria approved in the United States for adults.

The Prevnar vaccine is now only approved for children, but Pfizer has said an expanded population of adults could generate more than $1.5 billion in sales for what is already among the world's biggest selling vaccines.

Prevnar 13 protects against 13 strains of streptococcus pneumonia, a bacterium responsible for a range of ailments including meningitis, pneumonia and ear infections.

In the United States, the pneumococcal infection kills thousands of people each year, most of them 65 years of age or older, according to the Centers for Disease Control and Prevention.

The vaccine is poised to become one of Pfizer's biggest brands after the $10 billion-a-year cholesterol fighter Lipitor begins facing U.S. generic competition at the end of this month.

NEW GENERATION

The FDA said the older vaccine from Merck, known as a free polysaccharide vaccine, was effective against invasive pneumonia, but was not shown to have an effect on pneumococcal pneumonia, which is more common in adults.

The Pfizer vaccine is part of a new generation of pneumococcal vaccines known as conjugates that can protect against both types of pneumonia, said Emilio Emini, senior VP and chief scientific officer for vaccine research at Pfizer.

Such vaccines typically last longer and have a stronger immune response, he said.

The Prevnar franchise -- known as Prevenar in Europe and other countries -- generated about $3.7 billion in global sales last year.

The vaccine, acquired through Pfizer's purchase of rival U.S. drugmaker Wyeth in 2009, is currently approved for children six weeks to five years old.

A competing vaccine for children is Synflorix by GlaxoSmithKline , which protects against 10 strains of the bacteria that causes the pneumococcal infection.

Pfizer has submitted regulatory applications for use of Prevnar 13 in adults age 50 and older in more than 40 countries. The drug has already been approved for that use in the European Union, Australia, Bolivia, Colombia, Ecuador, Thailand and the Philippines, Pfizer said.